1. Field of the Invention
The present invention relates generally to a mobile communication system supporting a high-speed downlink packet access (HSDPA) scheme, and in particular, to an apparatus and method for acquiring a channel quality indicator (CQI).
2. Description of the Related Art
Currently, mobile communication systems are developing into high-speed, high-quality radio packet data communication systems for providing a data service and a multimedia service as well as a conventional voice service. Recently, for a high-speed, high-quality radio packet data service, standardization of 3rd generation mobile communication systems, which are classified into asynchronous 3GPP (3rd Generation Partnership Project) systems and synchronous 3GPP2 systems, is under way. For example, standardization of a high-speed downlink packet access (HSDPA) scheme is being conducted in 3GPP, while standardization of 1xEV-DV (Evolution Data and Voice) is being carried out in 3GPP2. Such standardizations are performed to find a solution for a high-speed, high-quality radio packet data service of 20 Mbps or higher in a 3rd generation mobile communication system, and a 4th generation mobile communication system is intended to provide a high-speed, high-quality multimedia service supporting a rate higher than that of the 3rd generation mobile communication system.
The HSDPA scheme refers to a high speed downlink shared channel (HS-DSCH) for supporting high speed downlink packet transmission in a code division multiple access (CDMA) mobile communication system, control channel related thereto, and a system as well as an apparatus and method therefore.
FIG. 1 schematically illustrates configuration of an asynchronous mobile communication system supporting an HSDPA scheme. Referring to FIG. 1, the asynchronous mobile communication system is comprised of a core network (CN) 100, a plurality of radio network subsystems (RNS) 110 and 120, and a user equipment (UE) 130. The RNS 110 includes a radio network controller (RNC) 111, and a plurality of Node Bs 115 and 113, and the RNS 120 also includes an RNC 112 and a plurality of Node Bs 114 and 116. Herein, for the convenience of explanation, the terms “Node B” and “cell” will be used in the same meaning.
The RNCs 111 and 112 are divided into a serving RNC (SRNC), a drift RNC (DRNC), and a controlling RNC (CRNC) according to their roles. The SRNC and the DRNC are classified according to the functions they support to their UEs. That is, an RNC that manages information on each UE and controls data transmission to the CN 100 is called an SRNC of a corresponding UE. When data from each UE is transmitted to the SRNC via another RNC, the RNC through which the data passes is called a DRNC of a corresponding UE. An RNC that controls each Node B is called a CRNC of a corresponding Node B. For example, in FIG. 1, if information on the UE 130 is managed by the RNC 111, the RNC 111 becomes an SRNC of the UE 130. Also, if data from the UE 130 is transmitted to the SRNC via the RNC 112 as the UE 130 moves, the RNC 112 becomes a DRNC of the UE 130. Further, the RNC 111 that controls the Node B 113 becomes a CRNC for the Node B 113.
The HSDPA scheme is a gathering of technologies for high-speed downlink data transmission in an asynchronous mobile communication system, and standardization thereof is being carried out in 3GPP, as stated above. However, many parts of the HSDPA scheme are still under discussion. Therefore, undefined subject matters of the HSDPA scheme will be described herein based on the discussion results up to the present.
In an asynchronous mobile communication system, high-speed downlink data transmission is realized with user of a plurality of OVSF (Orthogonal Variable Spreading Factor) codes, adaptive channel coding, and HARQ (Hybrid Automatic Retransmission Request). The HARQ can be realized with a fast retransmission and soft combining scheme. In the HSDPA scheme, the maximum number of OVSF codes that can be applied to one UE is 15, and QPSK (Quadrature Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation), and 64QAM (64-ary Quadrature Amplitude Modulation) are adaptively selected as modulation schemes according to a channel condition. In addition, defective data is retransmitted between a UE and a Node B, and then, the retransmitted data is soft-combined, thereby improving the entire communication efficiency. Such a retransmission scheme is called “n-channel SAW HARQ (Stop And Wait Hybrid Automatic Retransmission Request).”
The OVSF codes used to support the HSDPA scheme can be simultaneously used by a plurality of UEs. That is, the OVSF codes can be simultaneously multiplexed to the UEs. This will be described in more detail with reference to FIG. 2.
FIG. 2 illustrates a conventional method of assigning OVSF codes in a mobile communication system supporting an HSDPA scheme. In describing FIG. 2, it will be assumed that a spreading factor is 16 (SF=16).
Referring to FIG. 2, OVSF codes are represented by C(i,j) according to their positions in a code tree. In the C(i,j), i denotes spreading factor value and j denotes an order of a corresponding code from the leftmost side of the OVSF code tree. For example, C(16,0) indicates an OVSF code located in the first position from the left in a row with a spreading factor 16 (SF=16). FIG. 2 illustrates a method for assigning 15 SF=16 OVSF codes C(16,0) to C(16,14) in the OVSF code tree to a communication system supporting an HSDPA scheme. The 15 OVSF codes can be multiplexed to a plurality of UEs, and an example of the multiplexed OVSF codes is illustrated below in Table 1.
TABLE 1UE #AUE #BUE #Ct0C(16,0)~C(16,5)C(16,6)~C(16,10)C(16,11)~C(16,14)t1C(16,0)~C(16,3)C(16,4)~C(16,14)—t2C(16,0)~C(16,3)C(16,4)~C(16,5)C(16,6)~C(16,14)
In Table 1, UE #A, UE #B, and UE #C are UEs using a mobile communication system supporting the HSDPA scheme. As illustrated in Table 1, at times t0, t1, and t2, the UE #A, UE #B, and UE #C are code-multiplexed using OVSF codes assigned thereto. The number of OVSF codes assigned to the UEs and the positions of the OVSF codes in the OVSF code tree are determined by the Node B. The Node B determines the number and positions of the OVSF codes considering an amount of user data of each UE, stored in the Node B, and conditions of channels set up between the Node B and the UEs.
Therefore, control information exchanged between a UE and a Node B includes (i) code information indicating the number of OVSF codes to be used by a particular UE and positions of the OVSF codes in the code tree, (ii) channel quality information and modulation scheme information necessary for adaptively determining a modulation scheme according to a channel condition, and (iii) channel number information and ACK/NACK (Acknowledge/Negative Acknowledge) information necessary for supporting n-channel SAW HARQ.
Commonly, channels used in a mobile communication system to support an HSDPA scheme are classified into downlink channels and uplink channels. The downlink channels include a high-speed shared control channel (HS-SCCH), an associated dedicated physical channel (Associated DPCH), and a high-speed physical downlink shared channel (HS-PDSCH). The uplink channels include an uplink secondary dedicated physical control channel (HS-DPCCH).
Timing relations between the respective channels are illustrated in FIG. 3. A UE first measures channel quality between the UE itself and a Node B, using a primary common pilot channel (PCPICH), and then notifies the measurement result to the Node B using a channel quality indicator (CQI). The CQI is transmitted over the HS-DPCCH. The Node B then performs scheduling using the CQI. The scheduling determines a UE that will actually receive data for a next transmission time interval (TTI), among a plurality of UEs receiving an HSDPA service in the same cell. In addition, the scheduling refers to an operation of determining a modulation scheme to be used for corresponding data transmission and the number of codes to be assigned. When data transmission for a particular UE is determined, the Node B transmits control information necessary for receiving the corresponding data, over HS-SCCH. At this point, the UE can identify the HS-SCCH to be received, using a UE ID (Identifier). In addition, the UE needs to receive only a maximum of 4 HS-SCCHs considering complexity. However, a cell facilitates scheduling of packet data by managing 4 or more HS-SCCHs. A set of HS-SCCHs assigned to one UE is referred to as a “serving HS-SCCH set.” The serving HS-SCCH set can be designated according to UEs. Other details of this process will be described herein below.
Control information included in the HS-SCCH includes 7-bit information (code information) on OVSF codes to be used for HS-PDSCH, 1-bit information indicating a modulation scheme to be applied to HS-PDSCH, 6-bit information indicating a size of data transmitted over HS-PDSCH, and HARQ-related information. The HARQ-related information is comprised of a total of 7 bits: 1 bit for a new data indicator indicating whether data to be transmitted over HS-PDSCH is new data or not, 3 bits for a redundancy version (RV) of data to be transmitted over HS-PDSCH, and 3 bits for a channel number of data to be transmitted over HS-PDSCH in n-channel SAW HARQ.
FIG. 4 illustrates a conventional structure of the HS-SCCH. As illustrated in FIG. 4, the HS-SCCH is transmitted using an SF=128 OVSF code, and divided into 3 parts of Part-1, Part-2, and CRC. The 8-bit Part-1 information is transmitted over a first slot having 40 bits among the slots constituting an HS-SCCH frame, and the 13-bit Part-2 information and the 16-bit CRC are transmitted over second and third slots having 80 bits among the slots constituting the HS-SCCH frame. Because the Part-1 information and the Part-2 information are separately channel-coded in this way, a terminal can identify which of the 4 HS-SCCHs transmits control information for HS-PDSCH reception, by simply receiving only a first slot that transmits the Part-1 information.
The Part-1 includes code information indicting the number and positions of OVSF codes in the OVSF code tree, to be used by a corresponding UE, and modulation scheme information.
FIG. 5 illustrates a UE ID-based scrambling device for UE identification after channel coding Part-1 information and receiving the Part-1 information. Referring to FIG. 5, the Part-1 information is coded by an R=½ convolutional code (or a convolutional code with a rate=½), and then rate-matched to 40 bits corresponding to one slot. A 10-bit UE ID is coded with 32 bits by a (32,10) block code used for TFCI coding in Rel'99 standard, and then extended to 40 bits by repetition, which are XOR-ed with 40 bits of a first slot thereby performing scrambling according to UE IDs.
Part-2 includes transport block (TB) size information indicating a size of data transmitted over HS-PDSCH, a channel number of n-channel SAW HARQ, a new data indicator indicating whether corresponding data is new data or retransmitted data, and a redundancy version indicating a version of corresponding data in IR (Incremental Redundancy).
Finally, the CRC is filled with a CRC operation result for the Part-1 information and the UE identifier. It is expected that 10 bits are used for the UE identifier. That is, though not actually transmitted, the CRC is calculated along with the UE identifier in a transmission side. The CRC is calculated together with the UE identifier even in a reception side. By doing so, a UE can determine whether information contained in a particular HS-SCCH is information for the UE. For example, when transmitting control information to a particular UE #a using HS-SCCH, a Node B calculates CRC using Part-1, Part-2, and an identifier of the UE #a. When the UE #a calculates the CRC using its own UE identifier, Part-1 and Part-2 altogether, if no CRC error occurs, the UE #a determines that HS-SCCH transmits control information for the UE #a itself.
In order for a UE to receive the HS-SCCH, the UE receives control information necessary for HS-PDSCH reception by generating a scrambling sequence with a stored UE ID, performing descrambling on first slots of 4 HS-SCCHs, and then identifying HS-SCCH assigned thereto while performing Viterbi decoding on a convolutional code. After receiving the control information of HS-SCCH, the UE calculates CRC using Part-1 information, Part-2 information, and its own UE ID to determine whether an error has occurred. If no error has occurred, the UE continuously decodes HS-PDSCH information, determining that control information for the UE was received with error. However, if a CRC error has occurred, the UE suspends decoding of HS-PDSCH information.
Based on the information received over the HS-SCCH, the UE receives data transmitted over HS-PDSCH, demodulates the received data, and performs accordingly. At this point, the UE determines through which OVSF code it will receive HS-PDSCH, based on code information, and determines in which modulation scheme it will modulate the received HS-PDSCH, based on modulation information. After completion of this process, the UE determines through a CRC operation whether an error has occurred in corresponding data, and then transmits ACK/NACK information according to the CRC operation result. That is, if no error has occurred, the UE transmits an ACK, but if an error has occurred, the UE transmits a NACK.
The UE transmits ACK/NACK for packet data and CQI information for a downlink channel condition over HS-DPCCH. A structure of an HS-DPCCH is illustrated in FIG. 6. As illustrated in FIG. 6, a spreading factor of the HS-DPCCH is SF=256, and an HS-DPCCH subframe is comprised of 3 slots. The HS-DPCCH subframe transmits ACK/NACK information over its first slot, and transmits CQI information over its second and third slots. At this point, 1-bit ACK/NACK information is repeated 10 times, so the ACK/NACK information is transmitted with 10 bits. 5-bit CQI information is (20,5) channel-coded and transmitted with 20 bits.
When a UE supporting the conventional HSDPA scheme is located in a soft handover region, the UE uses an uplink power control method in which it determines whether it will transmit an HS-Pilot, according to a presence/absence of HS-PDSCH packet data. That is, when a UE supporting the conventional HSDPA scheme is located in a soft handover region, neighbor Node Bs transmit HS-PDSCH to the UE through a downlink. In order to enable a Node B receiving HS-DPCCH from the UE to efficiently perform channel compensation and power control on HS-DPCCH, an HS-Pilot can be inserted in the HS-DPCCH as is illustrated in FIG. 7. Therefore, the Node B can perform channel estimation, channel compensation and power control on the HS-DPCCH using the HS-Pilot, independently of an existing uplink DPCCH.
However, in the channel compensation and power control method using an HS-Pilot, when a UE is located in a soft handover region, even though there is no packet data transmitted over HS-PDSCH, the HS-Pilot is continuously transmitted, causing continuous uplink interference to a non-serving cell.
In order to solve this problem, the UE can transmit an HS-Pilot only in a period where it transmits ACK/NACK information after detecting the HS-SCCH, and then perform power control on the HS-DPCCH using the HS-Pilot. Although interference by transmission of HS-Pilot can be reduced by doing so, a CQI coding method must be changed to (20,5) and (15,5) coding methods according to whether the HS-Pilot is transmitted.
The method of changing a coding rate of the CQI information must be performed according to HS-SCCH detection at a UE. That is, upon detecting the HS-SCCH transmitted from a Node B, the UE encodes CQI information at a coding rate (15,5) before transmission, and upon failure to detect the HS-SCCH, the UE encodes the CQI information at a coding rate (20,5) before transmission.
However, there is probability that an HS-SCCH transmission/reception error will occur between a Node B and a UE. In this case, the UE fails to detect the HS-SCCH, and thus encodes CQI information at a coding rate (20,5) before transmission. In this situation, because a Node B has already transmitted HS-PDSCH packet data to a corresponding UE, the Node B expects that CQI information of the HS-DPCCH was encoded at a coding rate (15,5). Therefore, a coding rate and a decoding rate for CQI information between the UE and the Node B are mismatched, and the Node B cannot acquire correct CQI information. More specifically, when the HS-SCCH is not normally delivered from the Node B to the UE, CQI information cannot be correctly delivered from the UE to the Node B.